Substitution In Chocolate Cake Recipe

I am going to make this chocolate cake over the weekend. I found the recipe on Cake Central. I did not get butter milk and vegetable oil at my grocery. Can I substitute the buttermilk for homemade buttermilk (fullcream milk mixed with either lime juice, cream of tartar or vinegar) and soyabean oil instead?Also, I recently purchased an oven thermometer. When I used the thermometer for the first time. apparently I have to turn my oven on at its highest in order to get the desired temperature (350) which I did not get. I eventually had to use my own judgement adjusting the oven knob accordingly any tips/suggestions. Not sure if I am doing something wrong.

Ingredients2 cups of flour2 cups of sugar3/4 cup of cocoa2 tsp. baking soda1 tsp. of salt2 large eggs1 cup of buttermilk1 cup of vegetable oil1 1/2 tsp of vanilla extract1 cup of boiling water Instructions1. Preheat oven to 300 degrees (This is a slow bake cake)2. Lightly grease and flour you pan (9 x 13 in.)3. Place all the dry ingredients (except for cocoa) into a large bowl and combine.4. Then add the eggs, buttermilk, vegetable oil to the dry ingredients.5. Add the cocoa and the vanilla extract to the boiling water and add the mixture.6. Mix everything together at medium speed until everything is combined.7. Pour into pan and bake for 1 hour.

Yes, to the homemade buttermilk...do it all of the time with lemon juice or vinegar.I am pretty sure yes to the oil as well. I always use olive oil (the kind that says mild taste) and have never had a problem switching it out in any recipe. I think your choice would be fine.